The following abbreviations are herewith defined, at least some of which are referred to within the following description of the prior art and the present invention.
BPDU Bridge Protocol Data Unit
E-NNI External Network to Network Interface
L2GP Layer Two Gateway Port
MSTP Multiple Spanning Tree Protocol
NMS Network Management System
PBB Provider Backbone Bridging
PBBN Provider Backbone Bridge Network
RSTP Rapid Spanning Tree Protocol
The count-to-infinity problem is well known in the networking field and may appear after a topology change (e.g., failure event) in a network which is controlled by a distance vector protocol. To briefly explain the count-to-infinity problem, imagine a network connected with network nodes A-B-C-D-E-F where B-C-D-E-F form a ring and A is connected to this ring through B, Let the cost towards the root be “number of hops”, i.e. apply equal metric for each link. Now suppose that node A has a failure event. In the vector-update-process, the node B notices mat the route to node A, which was distance I, is down e.g. because node B does not receive the vector update from node A. The problem is, node B also gets an update from node C, and node C is still not aware of the fact that node A is down so it tells node B that node A is reachable through D, which is false. The false information slowly propagates through the network towards infinity until it ages out.
The networks which implement the Rapid Spanning Tree Protocol (RSTP) which is a well known distance vector protocol may experience the count-to-infinity problem in certain failure scenarios. The same holds for the networks that implement the well known Multiple Spanning Tree Protocol (MSTP) which utilizes multiple RSTP instances so it has the same principles as RSTP and is considered as an extension of RSTP which means it may also experience the count-to-infinity problem in certain failure scenarios. In addition, the networks which have a Provider Backbone Bridge Network (PBBN) interfaced with an External Network to Network Interface (E-NNI) in accordance with the Layer Two Gateway Port (L2GP) protocol may also experience the count-to-infinity problem. These networks can experience the count-to-infinity problem because the L2GP protocol is an extension of MSTP. The PBBN, E-NNI, and the L2GP are described in detail in IEEE Sid. 802.1 ah, “IEEE Standard for Local and Metropolitan Area Networks: Virtual Bridged Local Area Networks—Amendment 6: Provider Backbone Bridges,” 2008. The contents of this particular standard are incorporated herein by reference. FIG. 1 (PRIOR ART) is a diagram which illustrates the standard IEEE 801.1 bridge notation that is also used in this document.
As specified in IEEE 802.1 ah, each Layer Two Gateway Port (e.g., network interface port) is configured with a so called pseudoRootId, which is in fact a pseudo Bridge Identifier. The pseudoRootIDs are configured to be superior to any Bridge Identifier in the attached network domain. The Layer Two Gateway Ports then communicate their pseudoRootID in Bridge Protocol Data Units (BPDU) so the bridges in the domain of the attached PBBN interpret this pseudo information as real RSTP/MSTP information. In this manner, the Layer Two Gateway Ports emulate that they are connected to a bridge that is a potential candidate for being the root bridge of the spanning tree. The Layer Two Gateway Port which is configured with the superior pseudoRootID is the only one that remains active while all the other Layer Two Gateway Ports become blocked as the entire network considers that the root bridge is available through the one active Layer Two Gateway Port. If the connectivity is lost at the active link, then one of the formerly blocked links is activated. The order of precedence for the Layer Two Gateway ports is determined by their configured pseudoRootID. In this manner, the E-NNI is practically controlled by normal RSTP operation. An example of this type of system is provided and described below with respect to FIGS. 2A-2B (PRIOR ART).
Referring to FIG. 2A (PRIOR ART), there is a diagram illustrating an exemplary system 200 including a PBBN 202, an E-NNI 204, and a NMS 206 configured in accordance with IEEE 802.1 ah. The exemplary PBBN 202 has three bridges 208, 210, and 212 connected to one another plus the bridges 210 and 212 have network interface ports 2 and 4 (L2GPs 2 and 4) which are respectively connected to external bridges 214 and 216 within the E-NNI 204, Port 2 of bridge 210 and port 4 of bridge 212 are network interface ports (L2GPs) configured with different pseudoRootIDs 1 and 2 as specified in IEEE 802.1ah. The pseudoRootID 1 configured at port 2 of bridge 210 is “101”, which is the best (superior) in this scenario, therefore network interface port 2 (L2GP 2) is an active port. The pseudoRootID 2 of port 4 of bridge 212 is “202”, which is worse than (inferior to) port 2 thus port 4 (L2GP 4) is a blocked port. The NMS 206 is used to configure the pseudoRootIDs 1 and 2. At this point, there is no link failure between PBBN 202 and E-NNI 204 and the operation of L2GP is modeled in accordance with RSTP, thus different bridges 214 and 216 are needed to implement the different pseudoRootIDs 1 and 2.
Referring to FIG. 2B (PRIOR ART), there is a diagram illustrating how a link failure between the PBBN 202 and the E-NNI 204 results in the count-to-infinity problem. In this example, assume there is a link failure 218 between bridges 210 and 214 which causes the pseudoRootID 1 (i.e. “101”) configured at the port 2 (L2GP 2) of the broken link to be circulated in the entire attached PBBN 202 until it ages out and a root path cost 220 to root bridge 214 is increased in each circulation from 10 to 100 (for instance), which is the count-to-infinity. In this example, the root path cost 220 reaches 100 before it ages out. In this situation, the worst case RSTP/MSTP convergence appears, furthermore, the backup interface port 4 in bridge 212 cannot take over as it is blocked till the end of the count-to-infinity. To make matters worse, the protection switching at the E-NNI 204 due to the link failure 218 results in a new root bridge election procedure of RSTP, and then the set-up of a new spanning tree, which is the worst case RSTP operation, especially if it results in count-to-infinity. Hence, the protection switching at the E-NNI 204 results in the worst case convergence for the attached PBBN 202, which should not be affected at all in art ideal ease. These shortcomings and other shortcomings are addressed by the present invention.